1. Field of the Invention
This disclosure generally relates to the field of automatic data collection, and more particularly to machine-readable symbol readers operable to read machine-readable symbols, for example, bar code symbols.
2. Description of the Related Art
A variety of machine-readable symbol readers for reading machine-readable symbols are known. Symbol readers typically employ one of two fundamental approaches, scanning or imaging.
In scanning, a focused beam of light is scanned across the machine-readable symbol, and light reflected from and modulated by the machine-readable symbol is received by the reader and demodulated. With scanning type some readers, the machine-readable symbol is moved past the reader. With other scanning type readers, the reader is moved past the machine-readable symbol. Other scanning type readers move the beam of light across the machine-readable symbol while the reader and machine-readable symbol remain approximately fixed with respect to one another. Demodulation typically includes an analog-to-digital conversion and a decoding of the resulting digital signal.
In imaging, the machine-readable symbol reader may flood the machine-readable symbol with light, or may rely on ambient lighting. A one-dimensional (linear) or two-dimensional image capture device or imager such as a charge coupled device (CCD) array captures a digital image of the illuminated machine-readable symbol, typically by electronically sampling or scanning the pixels of the image capture device. The captured image is then decoded, typically without the need to perform an analog to digital conversion.
Scanning type readers typically employ a source of coherent, collimated light, for example a laser diode, to produce the beam. The scanning type reader may include a beam deflection system, for example a rotating or oscillating mirrors or prisms, to scan the resulting beam across the machine-readable symbol. To achieve high scan rates, the beam deflection systems must rotate or oscillate at very high frequencies. Such beam deflection systems are costly. Beam deflection systems also are prone to shock. Further, the use of moving parts in beam deflection systems, particularly parts that rotate or oscillate at high frequency, make such systems more susceptible to mechanical failure.
Imagers type readers are inherently more reliable than scanning type readers because they reduce or eliminate moving parts such as the rotating or oscillating mirrors or prisms. However, scanning type readers typically have a greater depth-of-field (i.e., range of distances between the symbol and the scanning type reader over which a symbol can be successfully decoded) than imaging type readers. An auto-focus mechanism may be added to the reader to enhance or improve the limited depth-of-field of the imaging type reader. The auto-focus mechanism typically includes a moveable optical element and either an active or passive range sensing subsystem. The range sensing subsystem causes the movement of the optical element to focus an image of the machine-readable symbol on the image sensor or imager. However, the addition of the movable optical element decreases the inherent reliability advantage that imaging type readers have over scanning type readers.
There is a need in the automatic data collection arts for a symbol reader that can overcome at least some of the aforementioned drawbacks.